Paper-like film and method for making it

ABSTRACT

A micro-voided film comprising high density polyethylene having a molecular weight of at least about 200,000, and low aspect ratio filler having a mean particle size from about 1 to about 25 microns. The film has a thickness of from about 10 to about 75 microns and a void fraction of from about 0.60 to about 0.75. The micro-voided film is made by a process comprising extruding the composition into a film having a thickness of from about 50 to about 300 microns, and orienting the extruded film using a high stalk, blown film process. The process produces a stabilized high stalk for increasing the production rate of blown, high molecular weight polyethylene, while increasing the film&#39;s physical and mechanical properties. The high stalk can be stabilized by application of high velocity, low volume flow rate of air over the interior and exterior surfaces of the extruded film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 13/004,840 (pending), filed Jan. 11, 2011 which claims the benefitof U.S. Provisional Application No. 61/294,372, filed Jan. 12, 2010, thedisclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a paper-like film comprising highdensity polyethylene and low aspect ratio, inorganic mineral fillermaterial, and a process for making the film. More particularly, theinvention relates to a micro-voided film made by extruding thecomposition through a circular die, and then orienting the film using ahigh stalk, blown film process.

BACKGROUND OF THE INVENTION

Synthetic papers comprising polyethylene and inorganic mineral fillersare known in the art. For example, U.S. Pat. No. 6,280,680, Liang, issaid to provide environmentally friendly paper comprising 56-80% ofinorganic mineral powders, 43-18% of polyethylene, and 1-2% ofadditives. The paper is made by a process using at least of one extruderand a forming mold having a circular die.

U.S. Pat. No. 4,606,879, Cerisano, discloses polymer films made using ahigh stalk, blown film extrusion process said to provide increasedproduction rates and improved film physical and mechanical properties.

Micro-voided films produced by stretching mineral filled polymercompositions are disclosed in WO 94/06849, Bergevin et al. The films aresaid to have paper-like qualities of opacity, whiteness andprintability, with improved flexural stiffness.

While the above paper-like films may be useful as described therein,there is a continuing need for thin, paper-like films that can beproduced at higher yields than typically obtained using prior artprocesses.

SUMMARY OF THE INVENTION

The present invention relates to a micro-voided film comprising, byweight, i) from about 20% to about 60% of high density polyethylenehaving a molecular weight of at least about 200,000, and ii) from about40% to about 80% of low aspect ratio filler having a mean particle sizefrom about 1 to about 25 microns, wherein the weight ratio of the lowaspect ratio filler to the polyethylene is at least about 0.7, said filmhaving a having a thickness of from about 10 to about 75 microns and avoid fraction of from about 0.60 to about 0.75.

The invention also relates to a process for making a micro-voided film,comprising: a) extruding a composition comprising, by weight, i) fromabout 20% to about 60% of high density polyethylene having a molecularweight of at least about 200,000, and ii) from about 40% to about 80% oflow aspect ratio filler having a mean particle size from about 1 toabout 25 microns, wherein the weight ratio of the low aspect ratiofiller to the polyethylene is at least about 0.7, into a film having athickness of from about 50 to about 300 microns, and b) orienting saidextruded film using a high stalk, blown film process, the resulting filmhaving a thickness of from about 10 to about 75 microns and a voidfraction of from about 0.60 to about 0.75.

In one embodiment, the invention relates to a process for making amicro-voided film, comprising: a) extruding a composition comprising, byweight, i) from about 20% to about 60% of high density polyethylenehaving a molecular weight of at least about 200,000, and ii) from about40% to about 80% of low aspect ratio filler having a mean particle sizefrom about 1 to about 25 microns, wherein the weight ratio of the lowaspect ratio filler to the polyethylene is at least about 0.7, into aprogressively advancing unexpanded tubular film having a thickness offrom about 50 to about 300 microns and having a substantially uniformfirst diameter about a cylinder arranged along a longitudinal axis overa predetermined distance, b) applying a first gas stream over theexterior surface of said tubular film, c) applying a second gas streamover the interior surface of said tubular film within an annular regionformed between said cylinder and the interior surface of said tubularfilm, d) controlling the velocity and volume flow rate of said first andsecond gas streams over said unexpanded tubular film over saidpredetermined distance for stabilizing said tubular film by preventingthe oscillation of said tubular film about said cylinder, and e)applying a third gas stream over the exterior surface of said tubularfilm having said first diameter adjacent the extent of saidpredetermined distance for stabilizing and expanding said tubular filmfrom said first diameter to a second diameter thereat, the resultingfilm having a thickness of from about 10 to about 75 microns and a voidfraction of from about 0.60 to about 0.75.

BRIEF DESCRIPTION OF THE DRAWING

The drawing schematically illustrates apparatus and a high stalk, blownfilm process useful to produce a paper-like film of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a high stalk, blown film extrusionprocess for making micro-voided, paper-like film. The features andadvantages of the invention will be more fully understood by referenceto the following detailed description, when taken in conjunction withthe accompanying drawing.

The film of the invention comprises, by weight, from about 20% to about60%, typically from about 30% to about 55%, more typically from about40% to about 50%, of high density polyethylene having a molecular weightof at least about 200,000. The term polyethylene means ethylenehomopolymers or copolymers made of ethylene and at least one otherolefin monomer. High density polyethylene means a polyethylene having adensity of at least about 0.940 g/cm³, typically from about 0.940 toabout 0.960 g/cm³. The high density polyethylene typically has amolecular weight of at least about 500,000, more typically at leastabout 750,000, e.g. about 1,000,000 or 1,500,000. The high densitypolyethylene typically has a molecular weight of less than about3,000,000, more typically less than about 2,000,000. In one embodiment,the high density polyethylene has a melt index of less than 0.2 dg/min,typically from about 0.01 to about 0.15 dg/min, more typically fromabout 0.02 to about 0.10 dg/min, e.g., from about 0.02 to about 0.06dg/min. As used herein, melt index is measured by the procedures ofA.S.T.M. D-1238-90b, and the density of polyethylene is measured by theprocedures of A.S.T.M. D-1505-85. Mixtures or blends of high densitypolyethylene, with or without other polymer materials, for example,medium or low density polyethylene or polypropylene, may be used. Theoptimum temperature for stretching the film will depend on theparticular polyethylene or blend of polyethylene selected. Whenstretching the film, the film temperature should be below thecrystalline melting point and above the line drawing temperature. Inpractice, the actual film temperature is not usually measured, asdescribed hereinafter.

The film further comprises, by weight, from about 40% to about 80%,typically from about 45% to about 70%, more typically from about 50% toabout 60%, of low aspect ratio filler material. The weight ratio of thelow aspect ratio filler to the high density polyethylene is at leastabout 0.7, typically at least about 0.8, more typically at least about0.9, e.g., about 1.0, 1.2 or 1.5. The term “aspect ratio” refers to theratio of particle length to particle thickness. For any given filler,the aspect ratio is the average value determined for a representativenumber of particles by examination through a microscope. The length isthe longest dimension, measured through the center of mass of theparticle. Once the length is known, it is possible to measure thedimensions of the particle in two other directions perpendicular to eachother and perpendicular to the length. These two dimensions are referredto as the width and thickness of the particle, with the thickness beingthe smaller of the two when they are not equal. In general, the lowaspect ratio fillers herein have an aspect ratio of less that about 3,typically less than about 2, more typically less than about 1.5. Fillerswith low aspect ratios, i.e., tending to the ratio of 1.0, althoughirregular, are often described as spherical, round, or cubic. Suitablelow aspect ratio fillers are selected from the group consisting ofalkali metal and alkaline earth metal carbonates, sulphates andphosphates, and mixtures thereof. Examples include calcium carbonate,sodium carbonate, barium sulphate, calcium sulphate, sodium sulphate,sodium phosphate, potassium phosphate, and calcium phosphate. In oneembodiment, the filler is calcium carbonate.

The particle size of the filler has an effect on the properties of thefilm. It is desirable that the fillers do not contain particles ofexcessively large size, otherwise holes or other defects may begenerated during the film stretching process. The maximum particle sizeof the filler depends on the desired film thickness. A.S.T.M. procedureE2651-10, Standard Guide for Powder Particle Size Analysis, may be usedto determine the size of the particles. When thicker films are to beproduced, larger particles can be tolerated. If the average particlesize of the low aspect ratio filler is too low, there is a tendency forthe resulting films to have lower void fractions.

In general, it is desirable that the low aspect ratio filler has amaximum particle size less than about 50 microns. It is also desirablethat at least 99.9% by weight of the filler particles pass through a 325U.S. mesh screen (nominal mesh openings of 44 microns). The low aspectratio filler herein has a mean particle size of from about 1 to about 25microns, typically from about 1 to about 20 microns, more typically fromabout 1 to about 10 microns. A desirable range for the mean particlesize, based on equivalent spherical diameter, for the low aspect ratiofiller is from about 1 to about 10 microns, typically from about 3 toabout 5 microns. Equivalent spherical diameter (ESD), the diametercomputed for a hypothetical sphere which would have the same volume asthe particle, is calculated as follows: ESD=(6×particle volume/π)^(1/3).

While not intending to be limited by theory, it is believed that thefilms of the invention have a structure with voids surrounding oradjacent to the low aspect ratio filler particles in the interior of thefilm. The smallest dimension of the low aspect ratio filler affects theformation of voids in the oriented film. If the particle size of thefiller is too small, the voids are absent or too small to give practicalpaper-like films. If the particle size of the filler is too large, thefilm tends to have holes therein, thus destroying the integrity of thefilm. For smaller particle size low aspect ratio fillers, it may bedesirable to add up to about 2% by weight of a C₁₀-C₂₄ organic acid, orblends thereof, before extruding the polymer-filler composition.Particularly when calcium carbonate is used, it is advantageous to addup to about 2% by weight, for example 1%, of a C₁₀-C₂₄ organic acid(e.g., coconut fatty acid, palmitic acid, or tallow fatty acid) to thecomposition.

The presence of microvoids in the film appears to manifest itself as anincrease in opacity and whiteness of the film compared to films withoutmicrovoids. There is also a noticeable reduction in density due to thefact that the film is no longer a uniform solid structure. Thisreduction in density can create difficulties when comparing film samplesof differing degrees of microvoiding. This may be further complicated byfilms that do not have smooth surfaces. For these reasons, additionalmeasurements are used for evaluating the films.

The density of a film sample is determined by measuring the length,width and average thickness, and determining its mass. Care must betaken not to overly compress the sample when measuring the thickness. Itis desirable to use a micrometer that applies only light force. Afterthe polyethylene and low aspect ratio filler components are compoundedtogether, the (compounded) resin density can be determined by using, forexample, a density column or other suitable method. The “void fraction”for the film sample can be calculated from the following formula: VoidFraction=1−(Film density/Resin density). It should be noted that thevoid fraction calculated in this way takes into account internal voidsand the effects of surface roughness. The films of the invention have avoid fraction of from about 0.6 to about 0.75, typically from about 0.62to about 0.72. With such a high void fraction, the films have arelatively low density, typically from about 0.40 to about 0.70 g/cm³,more typically from about 0.45 to about 0.65 g/cm³, e.g., from about0.50 to about 0.60 g/cm³.

“Equivalent thickness” is calculated as follows: Equivalentthickness=Thickness×(1−Void fraction). The equivalent thickness isintended to be a measure of the thickness that the film would have hadif compressed into a smooth, uniform, solid layer. Except whereexplicitly states as “equivalent thickness”, the term thickness refersto the measured thickness of the film and not the equivalent thickness.

The films of the invention can be produced at lower thicknesses, e.g.,down to about 10 microns, and at higher yields, e.g., up to about 600lbs/hr, than typically obtained using prior art processes. In variousembodiments, the present invention provides one or more of the followingbenefits: films having low density and low gauge (thickness) at standardproduction rates; films having equal tensile strength at half the gaugeof prior art films; films having equal moisture barrier properties athalf the gauge of prior art films; and films made with increasedthrough-put (significantly reduced lb/msi) versus prior art processes,e.g., a 100 micron thick film produced at 0.065 lb/msi versus a priorart film produced at 0.209 lb/msi, which translates into significantcost savings in the production of the film.

Films of the present invention combine the properties of both plasticand paper, specifically the moisture barrier, strength and elongation ofplastic and the tear and feel of paper. The films typically have lowextensibility and good flexural stiffness, die-cuttability, opacity,fold retention (good deadfold), and printability. As such, they areuseful as a paper substitute and as a replacement for plastic films andpaper currently used in flexible packaging and advertising signage. Thefilms can be used in flexible wrap or bag packaging for toiletries, suchas soaps, diapers, and tissues/wipes, and perishable and non-perishablefood products, such as cereals, grain-based foods and snacks. The filmscan be laminated to fiber based materials, such as fiber board,corrugated sheeting and box containers, and provide moisture barrierproperties and improved printing quality. The films can thus replacefoil lining in packaging and foil used in unitized bubble packaging forfood and medicinal products.

The films of the invention are thermo-degradable and photodegradablewhen exposed to ultraviolet light, and the inorganic filler materialsreturn naturally to earth in powder form, thus making the films moresustainable and environmentally friendly. The films reduce usage ofcostly polymer resin (produced from hydrocarbon fuels) in favor of lessexpensive inorganic filler materials. Manufacturing the film requiresless energy and no water usage compared to paper products. Manufacturingthe film also requires fewer steps and less energy than most plasticfilms due to the heat transfer properties of the inorganic fillermaterials and the low melt index of the high density polyethylene.

For certain applications, the film may be used without furthertreatment. For other applications, it may be desirable to includevarious additives conventionally used in the art, such as couplingagents, lubricants, dispersant agents, antistatic agents, antioxidants,processing aids, UV stabilizers and the like. Where the film is to beprinted, it may be desirable to corona treat the oriented film. Forother applications, e.g. films for in-mold labels, it may be desirableto corona treat and coat the film with an antistatic agent. Aftercoating with a heat seal layer, the films are particularly useful forin-mold labels since they comprise recyclable, high density polyethyleneresins. Heat seal layers are typically polyolefin materials having lowermelting points than the films of the invention. Ethylene vinyl acetate(EVA) copolymers are examples of such heat sealing layers.

The polyethylene and low aspect ratio filler materials of the inventionare usually first compounded by known methods for melt blendingthermoplastic polymers. For example, in a series of mixing, extruding,and milling steps, the polyethylene and low aspect ratio fillermaterials are compounded into pellets or granules having at leastsubstantially homogeneous composition. In one embodiment, the materialsare compounded into cylindrical shape pellets having a length anddiameter of about 3-5 mm. The pellets or granules are fed into anextruder, melted and extruded into a film using methods and apparatusknown in the art. The extruded film typically has a thickness of fromabout 50 to about 300 microns, more typically from about 200 to about300 microns. The film is then oriented, typically using stalk, blownfilm methods and apparatus known in the art. The resulting filmtypically has a thickness or from about 10 to about 75 microns, moretypically from about 20 to about 50 microns.

In one embodiment, the film is oriented as described in U.S. Pat. No.4,606,879, Cerisano, incorporated herein by reference. Cerisanodiscloses a blown film extrusion apparatus and process for producing astabilized, high stalk between spaced-apart tandem air rings forincreasing the production rate of blown polymers, while improving thefilm's physical and mechanical properties. The apparatus for forming thefilm is constructed of means for forming a progressively advancingtubular film along a longitudinal axis, stabilizing means arrangedexteriorly and interiorly of the film for preventing the oscillation ofthe film about the longitudinal axis over a predetermined distance, bycontrolling the application of a gas stream over the exterior andinterior surface of the film within the predetermined distance, andexpanding means arranged adjacent the boundary of the predetermineddistance for expanding the film thereat.

In another embodiment, the apparatus for forming the film is constructedof an extruder for supplying plastic material in a flowable state, a diearranged in advance of the extruder for forming a progressivelyadvancing tubular film along a longitudinal axis, a primary air ringarranged adjacent the die and exteriorly of the film, the primary airring supplying a first gas stream over the exterior surface of the film,a cylinder arranged interiorly of the film and extending along thelongitudinal axis from the die, the cylinder and the interior surface ofthe film defining an annular region between them for receiving a secondgas stream, controlling means for controlling the first and second gasstreams to stabilize the film over a predetermined distance bypreventing the oscillation of the film about the cylinder, and asecondary air ring arranged adjacent the boundary of the predetermineddistance and exteriorly of the film, the secondary air ring supplying athird gas stream over the exterior surface of the film for expanding thefilm thereat.

In another embodiment, the process for forming the film comprises thesteps of forming a progressively advancing tubular film along alongitudinal axis, stabilizing the film over a predetermined distance byapplying a gas stream over the interior and exterior surfaces of thefilm to prevent the oscillation of the film about the longitudinal axis,and expanding the film adjacent the boundary of the predetermineddistance.

In yet another embodiment, the process comprises the steps of extrudinga progressively advancing tubular film about a cylinder arranged along alongitudinal axis, applying a first gas stream over the exterior surfaceof the film, applying a second gas stream over the interior surface ofthe film within an annular region formed between the cylinder and theinterior surface of the film, controlling the velocity and volume flowrate of the first and second gas streams over a predetermined distanceto stabilize the film by preventing the oscillation of the film aboutthe cylinder, and applying a third gas stream over the exterior surfaceof the film adjacent the boundary of the predetermined distance forexpanding the film thereat.

Referring now to the drawing, there is shown a high stalk blown filmextrusion apparatus generally designated by reference numeral 100. Theextrusion apparatus 100 includes an extruder 102 having a supply hopper104 containing a polymer 106 (typically compounded granulates of thepolyethylene-filler material, or mixtures thereof optionally includingadditives as described below) to be blown into a thin film by theextrusion apparatus. The polymer 106 is heated to a molten state withinthe extruder 102 and forced under high pressure through an extrusion die108. The extrusion die 108 is circular in shape and has an annularopening through which a tubular film 110 of polymer in a semi-moltenstate is progressively advanced. The initial thickness of the tubularfilm 110 is determined by the size of the annular opening of theextrusion die 108. A primary air ring 112 is arranged adjacent theextrusion die 108 and surrounding the exterior of the tubular film 110.The primary air ring 112 is of the type known as a single lip air ringwhich prevents the performance of expansion work on the tubular film.The primary air ring 112 is connected to an air blower 114 via a conduit116. Arranged between the air blower 114 and primary air ring 112 is acontrol valve 118 and a temperature control unit 120. The control valve118 is adapted for controlling the velocity and volume flow rate of airfrom the air blower 114 to an opening 122 provided in the primary airring 112. The opening 122 is constructed and arranged for discharging acontinuous stream of air at uniform velocity and uniform rate in adirection parallel to the external surface of the tubular film 110.

A cylindrical mandrel 124 is positioned centrally overlying theextrusion die 108 and arranged along the longitudinal axis of thetubular film 110. The mandrel 124 is constructed to have a smoothuninterrupted exterior surface which defines an annular region 126 withthe interior surface of the tubular film 110 in the range of 0.125-1.4inches, typically in the range of 0.125-0.5 inches, and in oneembodiment, less than one-quarter inch. A passageway 128 is providedinternally of the mandrel 124 and communicates between the interiorregion 130 of the blown film 132 and a conduit 134 arranged underlyingthe extrusion die 108. The conduit 134 is connected to an exhaust blower136 which communicates with the surrounding atmosphere. A control valve138 is positioned in advance of the exhaust blower 136 to control therate of withdrawal of air from the interior region 130 of the blown film132. Optionally, a plurality of stabilizing guides 140 are positionedabout the exterior of the mandrel 124 and extend into the annular region126 to provide a restricted passageway 142. The guides 140 are arrangedabout the mandrel 124 at a location where the tubular film 110 hasattained sufficient mechanical strength by its partial solidification toprevent damage thereto in the event of contact with the guides. To thisend, the guides 140 are provided with a smooth uninterrupted exteriorsurface to prevent snagging of the tubular film 110. The guides 140provide for increased stabilization of the tubular film 110 by lockingthe tubular film thereat, as described hereinafter.

Air is supplied over the interior surface of the tubular film 110 withinthe annular region 126 by a single annular nozzle 144 arranged withinthe annular region overlying the extrusion die 108. The nozzle 144 isarranged such that the discharged air flows in an upward directionparallel to the interior surface of the tubular film 110. An air blower146 supplies air to the nozzle 144 through a conduit 148. A controlvalve 150 and temperature control unit 152 are provided within theconduit 148 between the air blower 146 and nozzle 144. The control valve150 and temperature control unit 152 function in the same manner as thecontrol valve 118 and temperature control unit 120 of the primary airring 112. The velocity and volume flow rate of air from the air blower146 is controlled by the control valve 150, while the temperature of theair is controlled by the temperature control unit 152. As described, thetemperature, the velocity and volume flow rate of a stream of airflowing in a parallel direction over the interior and exterior surfacesof the tubular film 110 may be controlled.

A secondary air ring 154 is arranged spaced-apart in tandem with theprimary air ring 112. The secondary air ring 154 is arranged apredetermined distance above the primary air ring 112 to define theextent of the tubular film 110 over which the tubular film isstabilized. The secondary air ring 154 can be adjusted upwardly anddownwardly by its support upon height adjustment member 156 as shown.The secondary air ring 154 is located adjacent the predetermineddistance over which the tubular film 110 is stabilized to provide alocation for film expansion. The secondary air ring 154 is of the duallip type adapted to perform substantial expansion work upon the tubularfilm 110 to provide the blown film 132. The secondary air ring 154 isprovided with a pair of spaced-apart openings 158, 160 for the dischargeof air at a high velocity and high volume rate as to opening 158 and ata high velocity, low volume rate as to opening 160 to create a negativepressure adjacent the exterior surface of the tubular film 110 toperform the required expansion work. Air is supplied to the secondaryair ring 154 by an air blower 162 connected to a conduit 164. A controlvalve 166 and temperature control unit 168 are arranged within theconduit 164 between the air blower 162 and secondary air ring 154. Thecontrol valve 166 controls the velocity and volume flow rate of airbeing discharged along the exterior surface of the tubular film 110 bythe secondary air ring 154, while the temperature control unit 168controls the temperature of the discharged air. Thus, the primary airring 112 is adapted for stabilizing the tubular film 110, while thesecondary air ring 154 is adapted for expansion of the tubular film toprovide the blown film 132. Although the primary air ring 112 performs amodest amount of controlled cooling of the tubular film 110, the primarycooling function is performed by the secondary air ring 154.

The extrusion apparatus 100 provides for increased stabilization of thetubular film 110 over a predetermined distance by the use of highvelocity, low volume flow rate of air discharged over both the interiorand exterior surfaces of the tubular film between the tandemly arrangedprimary air ring 112 and secondary air ring 154. The natural venturivector forces keep the external air next to the exterior surface of thetubular film 110, while the mandrel 124 maintains the low volume flowrate of air within the annular region 126 at a sufficiently highvelocity to keep the tubular film from oscillating about itslongitudinal axis. The guides 140 prevent buffeting and actually makegentle contact with the solidified inside surface of the tubular film110. The secondary air ring 154, being characterized as a high intensitycooling device, provides intensive cooling and expansion of the tubularfilm 110 at a location where desired, that is, providing the tubularfilm with a predetermined stalk height which is stabilized in accordancewith the present invention. The velocity of air inside the high stalk,outside the high stalk and at the high intensity cooling and expansionarea, i.e., the secondary air ring 154, are separately controlled tobalance and stabilize the tubular film 110 and blown film 132.

The extrusion apparatus 100 is adapted to manufacture blown film from avariety of high molecular weight polymers. In producing films of suchmaterial, solid polymer is provided in the supply hopper 104 to beextruded in a molten state through the extrusion die 108 by the extruder102. The thus formed tubular film 110 is stabilized over a predeterminedheight by the application of high velocity, low volume flow rate of airover the interior and exterior surfaces of the tubular film. The primaryair ring 112 applies such a stream of air over the exterior surface ofthe tubular film 110 at a controlled temperature by temperature controlunit 120 and at a controlled rate by control valve 118.

Similarly, a stream of air is applied over the interior surface of thetubular film 110 by the annular nozzle 144 at a controlled temperatureby temperature control unit 152 and at a controlled rate by controlvalve 150. The high velocity of the air stream over the interior andexterior surfaces of the tubular film stabilizes the film by preventingits oscillation about the mandrel 124 and about its longitudinal axis.The application of a low volume flow rate of air results in modestcooling of the unexpanded tubular film, thereby allowing for control ofthe stalk height to a predetermined distance.

As a result of this created stalk height, the amount of machinedirection drawdown of the tubular film 110 takes place at a reduced rateover that of conventional tubular film extrusion processes. Thispermitted relaxation of the polymer stresses within the high stalkprovides a uniformly stressed film for expansion having betteruniformity of thickness and physical and mechanical properties. Inaddition, the high stalk height allows for randomization andinterweaving of the long polymer molecules and the low aspect ratiofiller, rather than keeping them aligned parallel to the extrusiondirection. This randomization and interweaving gives the blown film 132improved tensile and tear strength properties and creates micro-voids inthe film. In addition, by controlling the temperature of the air streamsbeing applied over the exterior and interior surfaces of the tubularfilm 110, the film temperature over the high stalk may be maintained atan optimum temperature for ultimate blowing by the secondary air ring154, and while being stabilized. This stabilization of the high stalk isfurther enhanced by the guides 140 which create the narrow passageways142 to increase the velocity of air flowing in the annular region 126.This increased velocity of air has the tendency of locking the tubularfilm 100 about the guides 140, thereby increasing the stabilization ofthe tubular film over the predetermined distance of the thus createdhigh stalk.

The size of the bubble of the blown film 132 is controlled primarily bythe exhaust blower 136 and control valve 138. Generally, under steadystate operation, the mass in of air via air blower 146 is equal to themass of air being extracted from the interior region 130 by the exhaustblower 136 through the passageway 128 extending through the mandrel 124.In order to increase or decrease the size of the blown film 132, theinternal pressure within the interior region 130 is momentarilyincreased or decreased, so as to affect the size of the blown film 132,which size is sensed by means of sonar sensors 170. Once the blown film132 has achieved its predetermined size, the mass in and mass out of airwithin the interior region 130 is again balanced for steady stateoperation. The tubular film 110 is expanded and intensely cooledadjacent the frost line 172 in a conventional manner using the secondaryair ring 154.

As described, the extrusion apparatus 100 and method of manufacturingblown tubular film provide for increased bubble stability, improvedgauge uniformity, reduced gauge standard deviation, improved opticalproperties, improved impact strength, improved tear strength, improvedtensile strength, improved down gauging capability, and increasedoutput.

After the blown film 132 is formed, the film may be further processed byvarious means known in the art. For example, the film may be processedas shown in FIG. 2 of U.S. Pat. No. 6,280,680, Liang, incorporatedherein by reference. In one embodiment, one end of the film 132 is drawnby a leading roller, such as shown in FIG. 2 of the Liang patent. Therotation speed of the leading roller is controlled so that the film 132is substantially air-tight. The rotating speed of the leading roller,the amount of the extruded materials from the extruder 102, and thethickness of the film 132 are suitably controlled so that the film isinflated to a desired lay flat width relative to the blow ratio and diediameter at a distance of about 1000 mm to about 1700 mm from theextrusion die 108. The purpose of the inflation and drawing is tosimultaneously stretch the film 132 in two directions, i.e.,latitudinally and longitudinally, resulting in a paper-like film havinga structure with two dimensional strength. Under inflation, the densityof the paper-like film can be reduced from that of the combination ofthe raw materials, about 2 g/cm³ to about 0.5 g/cm³. Because the drawingforce from the leading roller, the film 132 is drawn into a foldingmeans provided between the leading roller and a cooling means so thatthe paper-like film is symmetrically folded into a folded flat paper.The purposes of the leading roller include drawing the initially formedfilm 132 with a low rotation speed so that the air from the coolingmeans is evenly blown thereto and stabilizing the film 132, andmaintaining the film air-tight so that it is evenly inflated. Also, therotation speed of the leading roller is a factor in the longitudinalstretch and thickness of the paper-like film. Of course, the rotationspeed should be suitably adjusted to comply with the amount of extrudedmaterials coming from the extruder 102. The folded paper typicallypasses a cutting means so that the folded paper is cut, e.g., into twosheets of paper. The two resulting sheets of the paper may be subjectedto treatment of a surface corona and better adhesion can be obtainedtherefrom. The paper may then be collected on a roll. During theprocess, the thickness of the paper can be suitably controlled within arange from about 25 microns to about 75 microns, the width can be about35-60 inches (about 0.9 m to 1.5 m), and the density can be about 0.4g/cm³ to about 0.7 g/cm³.

The present invention may also be used in a process for the manufactureof a double-layered paper, triple-layered paper, or higher layeredpaper, such as disclosed in U.S. Pat. No. 6,280,680, Liang. Such layeredpapers can be used for printing, packaging, and decoration, etc. Eachlayer can be designed with different colors as required by addingdifferent pigments thereto. Single-layered papers, double-layeredpapers, triple-layered papers, and even papers having more than threelayers, each independently having a thickness of from about 30 to about150 microns and independently having the same or different components,can be laminated by a laminating machine, such as shown in FIG. 8 of theLiang patent, to form a two-layered laminated paper or a three-layeredlaminated paper having a thickness of from about 150 to about 450microns.

The papers manufactured according to the present invention can beapplied to the field of printing, packaging, and decoration. They can beused directly without any pretreatment or with suitable pretreatment,for instance glossy surface treatment and hazy surface treatment, forspecial purposes. Both water borne coatings and non-water borne coatingscan be used to coat the papers manufactured using the present invention.The formulation of water borne coating may be comprised of acrylicresin, isopropanol, polyvinyl alcohol, clays, an antistatic agent, 28%aqueous ammonia, pure water, and vinyl acetate.

The following Table 1 discloses ranges for various operating parametersin producing blown tubular film of the invention using the apparatus andprocess described above. The operating parameters are based on the useof a 300 mm (11.8 inches) diameter extrusion die. The mass flow, insideair flow, outside air flow and secondary air ring flow are directlyproportional to the die diameter. Thus, for a 200 mm (8 inch) diameterextrusion die, the mass flow would be 200-1200 lb/hr, the inside airflow would be 40-1200 CFM, the outside air flow would be 40-1600 CFM,and the secondary air ring flow would be 120-4000 CFM.

TABLE I UNITS RANGE Resin Melt Index dg/min. 0.02-0.15  Melt Temp. ° F.375-450  Melt Pressure Psi 7000-9000  Mass Flow Lb/hr 300-900  Melt TubeDia. Inches 3.2 to 6   Melt Tube Thick Mils  20 to 200 Inside Air FlowCFM  60-1800 Inside Air Velocity FPM 1,000-24,000 Inside Air Temp. ° F.−20 to 300 Outside Air Flow CFM  60-2400 Outside Air Velocity FPM1,000-24,000 Outside Air Temp. ° F. −20 to 300 Sec. Air Ring Flow CFM180-6000 Sec. Air Ring Velocity FPM 2,000-24,000 Sec. Air Ring Temp. °F. −20 to 150 Blow Up Ratio — 0.8 to 9.0 Film Thickness Mils 0.1 to 20 Film Speed FPM   30 to 1,000 Melt Tube Height/Die — 1-20 dia.

Micro-voided, blown tubular films of the invention comprising, byweight, 50% high density polyethylene (molecular weight about 1.0 to 1.5million) and 50% calcium carbonate (mean particle size about 3-5microns) are produced using the above described apparatus and processunder the following conditions.

TABLE II EXAMPLES UNITS I II III Resin Type HDPE HDPE HDPE Low AspectRatio filler CaCO3 CaCO3 CaCO3 Resin Melt Index dg/min. .06 .04 .02 MeltTemp. ° F. 450 450 450 Die Diameter mm 300 200 150 Melt Pressure Psi8,000 6,000 4,000 Mass Flow Lb/hr 800 550 300 Inside Air Flow CFM 900600 300 Inside Air Temp. ° F. 45 45 45 Outside Air Flow CFM 1200 800 400Outside Air Temp. ° F. 45 45 45 Sec. Air Ring Flow CFM 3000 2000 1000Sec. Air Ring Temp. ° F. 45 45 45 Blow Up Ratio — 2.8 2.8 2.8 FilmThickness Microns 38 38 38 Film Speed FPM 200 150 100 Void Fraction —0.62 0.62 0.62

Other films of the invention are obtained when the above compositioncomprises, by weight, 40% high density polyethylene (molecular weightabout 1.0 to 1.5 million) and 60% calcium carbonate (mean particle sizeabout 3-5 microns), or 30% the high density polyethylene and 60% calciumcarbonate, or when about 1% of C₁₀-C₂₄ organic acid is added to thecomposition. For example, one film comprises about or 28% the highdensity polyethylene, 70% calcium carbonate, and 1 to 2% of additives,such as the C₁₀-C₂₄ organic acid. Other films of the invention areobtained when 10% or 20% medium density polyethylene or polypropylene isadded to the above composition, or when the calcium carbonate has a meanparticle size of about 10 microns or is replaced with barium sulphate,sodium sulphate, sodium phosphate, or calcium phosphate. Other films areobtained when the above films have a void fraction of about 0.65, 0.70or 0.75, or a density of about 0.40, 0.50, or 0.65 g/cm³, and athickness of about 10, 25 or 50 microns.

Although the invention has been described with reference to particularembodiments, it is to be understood that these embodiments are merelyillustrative of the principles and application of the invention, andthat the invention may be carried out in other ways than those set forthherein. For example, air from the interior region 130 of the blown film132, which is supplied by air blower 146, could be recirculated throughconduits 134, 148 thereby eliminating the exhaust blower 136 and controlvalve 138. Therefore, numerous modifications may be made in theillustrative embodiments and that other arrangements may be devisedwithout departing from the spirit and scope of the invention as definedby the appended claims.

What is claimed is:
 1. A micro-voided film comprising, by weight, i)from about 20% to about 60% of high density polyethylene having amolecular weight of at least about 500,000 and a melt index of less than0.2 dg/min, and ii) from about 40% to about 80% of low aspect ratiofiller having a mean particle size from about 1 to about 25 microns,wherein the weight ratio of the low aspect ratio filler to thepolyethylene is at least about 0.7, said film having a having athickness of from about 10 to about 75 microns and a void fraction offrom 0.60 to about 0.75 and a density of from about 0.40 to about 0.70g/cm³, wherein the voids are in the interior of the film.
 2. Themicro-voided film according to claim 1 wherein the film thickness isfrom about 20 to about 50 microns.
 3. The micro-voided film according toclaim 1 wherein the film has a void fraction of from about 0.62 to about0.72.
 4. The micro-voided film according to claim 1 comprising fromabout 30% to about 50% of the high density polyethylene.
 5. Themicro-voided film according to claim 1 wherein the high densitypolyethylene has a melt index of from about 0.02 to about 0.06 dg/min.6. The micro-voided film according to claim 1 wherein the low aspectratio filler has a mean particle size of from about 1 to about 10microns.
 7. The micro-voided film according to claim 1 comprising fromabout 50% to about 60% by weight of the low aspect ratio filler.
 8. Themicro-voided film according to claim 1 wherein the low aspect ratiofiller is calcium carbonate.
 9. The micro-voided film according to claim8 wherein the calcium carbonate has a mean particle size of from about 3to about 5 microns.
 10. The micro-voided film according to claim 1further comprising up to about 2% by weight of C₁₀-C₂₄ organic acid. 11.The micro-voided film according to claim 1 wherein the weight ratio ofthe low aspect ratio filler to the polyethylene is at least about 0.9,the low aspect ratio filler has a mean particle size of from about 1 toabout 10 microns, and the film thickness is from about 20 to about 50microns.